Energy resolving detectors for Computed Tomography rely on direct conversion materials. The pixel size of such detectors is determined by the trade-off between the count rate capability per unit area measured in Mcps (Million counts per second) per mm2 and the spectral information that can be resolved. That is, a smaller pixel will result in higher achievable count rate density at the cost of a reduced energy resolution. The energy resolution degradation is largely caused by charge sharing across pixels: part of the charge corresponding to an interaction in a given pixel volume drifts to the neighboring pixel, which results in an error in the energy estimation of the primary photon and uncertainty about the location of the interaction.
Charge sharing occurs when charges pertaining to a pixel drift in the electric field lines corresponding to a neighbor pixel. Since the charge cloud has a finite non-zero volume, it expands as it drifts towards the anode, due to e.g. diffusion. As there are no physical boundaries across pixels, this effect cannot be avoided unless some pixel confinement mechanisms are in place.
Charge sharing correction methods do exist in the electronics responsible to acquire the generated charge as a response to an impinging x-ray photon, e.g. R. Ballabriga et al, “The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode With Improved Spectrometric Performance” Nuclear Science, IEEE Transactions on Volume:54, Issue: 5. Such methods however pose a significant penalty in terms of count rate capabilities, as for each impinging photon, any neighbor event needs to be evaluated and arbitrated in order to locate the full charge in the pixel of origin.
Machining of direct converters has been proposed in the past. This however entails very intrusive methods such as sawing, milling and/or etching to be performed in a very brittle crystal, e.g. CdTe, CdZnTe, with severe consequences in terms of formation of large defects or even complete rupture. Creating a waveguide structure on substrate by utilizing FIB milling/etching is known from MoberlyChan et al. (MRS BULLETIN ● VOLUME 32 ● MAY 2007).
Alternative technologies have been developed to try to solve this problem. For instance, US20110211668 teaches to confine pixels by manipulating crystal growth. It is also known from US20140048714 to confine charge sharing between pixels by inducing Electric field rather than breaking, etching or milling fine recesses on the crystal.
The purpose to the invention is to provide a way to physically confine the pixels of an existing crystal in order to avoid charge-sharing issues.